1) Field of the Invention
The present invention relates to a method and apparatus for compensation between cross-polarized waves (hereinafter referred to as "inter-cross wave compensation"), and more particularly to a method and apparatus for inter-cross wave compensation suitable for use in digital radio transmission systems such as those used for digital multiplex radio communication.
2) Description of the Related Art
In radio transmission systems using microwaves, or quasi-millimeter waves, a technique of using both cross-polarized waves is generally used in which signals are transmitted using two polarized waves (a vertical (V) wave and a horizontal (H) wave) having the same frequency and respectively propagating along polarization planes intersecting each other perpendicularly. Such a cross-polarization technique is used because it becomes possible to more efficiently use such frequencies compared to the case where a single polarized wave is used for transmission.
FIG. 23 shows an example of the structures of the transmitting side and the receiving side of a general radio transmission system utilizing cross-polarized waves. In FIG. 23, numeral 100 denotes the transmitting side while numeral 200 denotes the receiving side. The transmitting side 100 comprises modulating units (MOD) 101A and 101B for V and H waves respectively, transmitting units (TX) 102A and 102B, a hybrid (H) 103 for mixing the V and H waves, and a transmitting antenna 104. The receiving side 200 comprises a receiving antenna 201, a hybrid (H) 202 for splitting received polarized signals into a V wave signal and an H wave signal, receiving units (RX) 203A and 203B for V and H waves respectively, and demodulating units (DEM) 204A and 204B.
In the transmitting side 100, data (DATA) undergo a predetermined modulation such as QAM (Quadrature Amplitude Modulation) by the modulating unit (MOD) 101A and the transmitting unit (TX) 102A, so that a V wave is obtained. Another set of data undergo a predetermined modulation by the modulating unit (MOD) 101B and the transmitting unit (TX) 102B, so that an H wave is obtained, having the same frequency as that of the V wave and intersecting the V wave. These V and H waves are mixed by the hybrid (H) 103 and are transmitted through the transmitting antenna 104. In the receiving side 200, modulated transmission signals transmitted from the transmitting side 100 are received by the receiving antenna 201 and are split into a V wave and an H wave by the hybrid 202. These waves are demodulated by the demodulating units 204A and 204B so as to reproduce the original data.
Generally, demodulation at the demodulating unit 204A (204B) of the receiving side 200 is performed by an analog demodulation method or a digital demodulation method. In the analog demodulation method, the modulated transmission signal, which is an analog signal, is detected to carry out demodulation of the signal. In the digital demodulation method, a digital signal contained in the modulated transmission signal is detected by an A/D (analog/digital) converter to carry out demodulation of the signal.
FIG. 24 is a block diagram showing an example of the structure of a demodulating unit 204A (204B) employing an analog demodulation method. The demodulating unit 204A (204B) shown in FIG. 24 comprises a mixer (MIX) 205, roll-off filters 206A and 206B, a transversal (TRV) equalizer 208, a control unit (CONT) 209, low pass filters 210 and 212, a voltage controlled oscillator (CLK VCO) 211 for supplying clocks (CLK) for the A/D converters 207A and 207B, and a local oscillator (LO VCO) 213 for the mixer 205.
A modulated transmission signal within an RF (radio frequency) band received by the receiving antenna 201 is subjected to frequency conversion by the receiving unit 203A (203B) to obtain an IF (intermediate frequency) signal. The reception IF signal is orthogonally detected using a carrier wave regeneration signal from the local oscillator 213, so that the frequency of the reception IF signal is converted (down conversion) so as to obtain two base band signals I and Q having a phase difference of 90.degree. therebetween. In practice, the reception IF signal is split into two waves by the hybrid 214. The split waves are multiplied (mixed) by multipliers 215 and 216 using signals from a hybrid 217, which splits the signal from the local oscillator 213 into two waves, so that the base band signals I and Q are obtained.
The roll-off filters 206A and 206B remove noise components and the like included in the base band signals I and Q, which have been obtained in the above-described manner. The A/D converters 207A and 207B convert the base band signals (analog signals) into corresponding digital signals I and Q in accordance with the timing of A/D conversion clocks supplied from the voltage controlled oscillator 210. In some cases, the roll-off filters 206A and 206B may be disposed on the output side of the demodulating unit 204A (204B) or in a stage succeeding the A/D converters 207A and 207B.
As is well known, the transversal equalizer 208 delays the digital signals I and Q to adaptively infer from the past data a distortion component (distortion produced by a transmission line) contained in present data, and controls an internal tap coefficient based on the inferred distortion component. With this operation, the distortion component is removed to equalize the signals.
The control unit 209 detects from the output of the transversal equalizer 208 information such as a shift of clock phase and a frequency error, and performs feedback control for the voltage controlled oscillator 211 and the local oscillator 213 based on the detected information. With this operation, frequency conversion and A/D conversion can be performed at an optimal clock phase and frequency. In other words, the demodulating unit 204A (204B) shown in FIG. 24 employs a so-called homodyne detection method.
The low pass filters 210 and 212 integrate information (digital values) such as a phase shift and a frequency error obtained by the control unit 209 so as to convert the information into voltage values. The voltage values are supplied to the voltage controlled oscillator 211 and to the local oscillator 213 as control voltages.
In the above-described demodulating unit 204A (204B), due to the above structure, the reception IF signal is subjected to orthogonal detection (homodyne detection) by the mixer 205 in accordance with the carrier wave regeneration signal from the local oscillator 213, so that the IF signal is converted to a lower frequency so as to obtain base band signals I and Q. The noise components and the like of the base band signals I and Q are removed by the roll-off filters 206A and 206B.
The base band signals I and Q are converted (A/D conversion) into the digital signals I and Q with an optimal phase by the A/D converters 207A and 207B in accordance with A/D conversion clocks from the voltage controlled oscillator 211, which is feedback-controlled by the control unit 209. The digital signals I and Q undergo an equalizing process performed by the transversal equalizer 208, so that the digital signals I and Q are outputted as reception data of the V wave (H wave).
FIG. 25 is a block diagram showing an example of the structure of a demodulating unit 204A (204B) employing an digital demodulation method. The demodulating unit 204A (204B) shown in FIG. 25 comprises a-down (DOWN) converter 221, an A/D converter 222, a digital orthogonal detection unit 223, roll-off filters 224A and 224B, a transversal (TRV) equalizer 225, a control unit (CONT) 226, low pass filters 227 and 228, a voltage controlled oscillator (CLK VCO) 229 for supplying clocks (CLK) for the A/D converter 222, and a local oscillator (LO VCO) 230 for the down converter 221.
The down converter 221 performs frequency conversion (down conversion) using the carrier wave regeneration signal received from the local oscillator 230 for the RF signal received by the receiving antenna 201 so as to obtain an IF (intermediate frequency) signal. Generally, the down converter 221 comprises a single mixer (multiplier). The A/D converter 222 converts the reception IF signal into a digital signal in accordance with timing clocks for A/D conversion supplied from the voltage controlled oscillator 229.
The digital orthogonal detection unit 223 performs orthogonal detection for the digital signal obtained by the A/D converter 222 using trigonometric function signals representing a sine (sin) wave and a cosine (cos) wave. As a result, frequency conversion is performed, and two digital signals I and Q in the base band range having a phase difference of 90.degree. therebetween are obtained. The roll-off filters 224A and 224B remove noise components and the like from the base band signals I and Q which have been obtained in the above-described manner.
The transversal equalizer 225, the control unit 226, the low pass filters 227 and 228, the voltage controlled oscillator 229, and the local oscillator 230 are identical to the transversal equalizer 208, the control unit 209, the low pass filters 210 and 212, the voltage controlled oscillator 211, and the local oscillator 213, respectively, which have been described with reference to FIG. 24. The homodyne detection method is also employed in the demodulating unit 204A (204B) using the digital demodulation method.
In the demodulating unit 204A (204B) employing the digital demodulation method, due to the above-described structure, a modulated wave (reception IF signal) which has been modulated by the modulating unit 101A (101B) of the transmitting side 100 using a carrier wave signal having a center frequency f.sub.LO, for example, is subjected to down conversion by the down converter 221, As a result, the modulated wave (reception IF signal) is converted (down conversion) such that it has a clock frequency (center frequency: f.sub.CLK) at which A/D conversion by the A/D converter 222 can be performed.
The reception IF signal is converted (A/D conversion) into a digital signal at an optimal phase by the A/D converter 222 in accordance with timing clocks for A/D conversion from the voltage controlled oscillator 229, which is feedback-controlled by the control unit 226. The digital signal is orthogonally detected by the digital orthogonal detection unit 223 to obtain digital signals I and Q of the base band having a phase difference of 90.degree. therebetween
Specifically, in the digital orthogonal detection unit 223, the digital signals I and Q are obtained by the following equations: EQU I=IF.sub.IN .times.cos.theta. EQU Q=IF.sub.IN .times.sin.theta.
wherein IF.sub.IN is the digital signal from the A/D converter 222 and .theta. is a clock phase, and wherein .theta. is varied over one cycle at the clock speed of the voltage controlled oscillator 229.
For example, assuming that the conversion rate of the A/D converter 222 is four times the clock speed, .theta. is repeatedly changed as follows: EQU 0.degree..fwdarw.90.degree..fwdarw.180.degree..fwdarw.270.degree..fwdarw.0. degree..fwdarw.90.degree..fwdarw.
Accordingly, the digital demodulated signals I and Q vary such that I=IF.sub.IN and Q=0 when .theta.=0.degree., I=0 and Q=IF.sub.IN when .theta.=90.degree., I=-IF.sub.IN and Q=0 when .theta.=180.degree., and I=0 and Q=-IF.sub.IN when .theta.=270.degree..
After that, the noise components and the like of the digital signals I and Q are removed by the roll-off filters 224A and 224B. The digital signals I and Q are then subjected to equalization by the transversal equalizer 225 and are outputted as reception data of the V wave (H wave).
As described above, in the demodulating unit 204A (204B) utilizing the digital demodulation method, the process by the A/D converter 222 and the following processes are all performed in digital form. Accordingly, the bulk of the demodulating unit 204A (204B) can be formed by digital circuits.
In the radio transmission system utilizing the technique of using both cross-polarized waves which has been described with reference to FIG. 23, it is required to provide good separation between two polarized waves, i.e., V and H waves (XPD: Cross Polarization Discrimination).
However, a signal including only a V wave (H wave) is interfered by an H wave (V wave) in a transmission pass such as a space, resulting in deterioration of XPD, Such an interference occurs, for example, due to multipath fading in the microwave band, and inclination of rain drops in the quasi-millimeter wave or higher wave band. Therefore, it is general practice to exchange V and H waves between the demodulating units 204A and 204B, as shown in FIG. 23, so as to perform compensation for interference between the polarized waves (inter-cross wave interference compensation), thereby compensating the deterioration of XPD.
FIG. 26 is a block diagram showing an example of the structure of a demodulating unit of an analog demodulation type which is used when the inter-cross wave interference compensation is performed. In FIG. 26, units and components identical to those shown in FIG. 24 are denoted by the same reference numerals. In the demodulating unit 204A (204B) shown in FIG. 26, analog V wave (H wave) signals (base band signals I and Q), before undergoing A/D conversion by the A/D converter 207A and 207B, are outputted to the demodulating unit 204B (204A) for the other polarized wave, i.e., the H wave (V wave), as a signal for the inter-cross polarized wave interference compensation. The demodulating unit 204A (204B) receives the H wave (V wave) signal from the demodulating unit 204B (204A) for the H wave (V wave), as a signal for the inter-cross polarized wave interference compensation.
In the demodulating unit 204A (204B), H wave (V wave) signals are subjected to A/D conversion by A/D converters 231A and 231B so as to obtain digital H wave (V wave) signals. Thus obtained digital signals are supplied to an inter-cross wave interference compensating unit (XPIC) 232, which detects H wave (V wave) signals mixed into the V wave (H wave) signals as an interference component.
The inter-cross wave interference compensating unit 232 outputs compensation signals for compensating the inter-cross wave interference to adders 234 and 235 of an adding unit 233. The compensation signals and the outputs of the transversal equalizer 208 are added together by the addition unit 233 to compensate the inter-cross wave interference.
The inter-cross wave interference compensating unit 232 generally comprises an FIR filter which is one kind of the transversal equalizer 208 and generates compensation signals by varying the internal tap coefficient in accordance with the magnitude of the inter-cross wave interference.
However, in the demodulating unit 204A (204B) of the analog demodulation type capable of compensating inter-cross wave interference, which has been described with reference to FIG. 26, only limited portions such as the transversal equalizer 208 and the inter-cross wave interference compensating unit 232 can be digital circuits. This is very unfavorable to efforts in reducing the size of the circuits and costs.
Further, to perform the above-described inter-cross wave interference compensation, wave signals for compensation which are exchanged between the demodulating units 204A and 204B must be synchronous with interference in the space.
FIG. 27 shows an example of the relationship between frequencies in the case where a so-called co-channel transmission (transmission using V and H waves) is performed in the structure shown in FIG. 23. In FIG. 27, units and components identical to those shown in FIG. 23 are denoted by the same reference numerals. Numerals 105A, 105B, 106A, 106B, 241A, 241B, 242A and 242B denote voltage controlled local oscillators
In the transmitting side 100, the local oscillators 105A and 105B supply the modulating units (MOD) 101A and 101B with carrier wave signals (center frequency: f.sub.V1, f.sub.H1) which are used for frequency conversion (up conversion, see functions (21) and (26) in FIG. 27) of transmission data (V and H waves) of the base band range into IF signals. The local oscillators 106A and 106B supply the transmitting units (TX) 102A and 102B with carrier wave signals which are used for up conversion (see functions (22) and (27)) of the IF signals from the modulating units 101A and 101B into RF signals.
In the receiving side 200, the local oscillators 241A and 241B supply the receiving units (RX) 203A and 203B with carrier wave regeneration signals (center frequency: f.sub.V3, f.sub.H3) which are used for frequency conversion (down conversion, see functions (24) and (29)) of reception signals (V and H waves, see functions (23) and (28)) received by the receiving antenna 201 into IF signals. The local oscillators 242A and 242B supply the demodulating units 204A and 204B with carrier wave regeneration signals (center frequency: f.sub.V4, f.sub.H4) which are used for down conversion (see functions (25) and (30)) of the IF signals from the receiving units 203A and 203B into base band signals.
In other words, the local oscillators 242A and 242B correspond to the local oscillators (LO VCO) 213 or 230 of the demodulating unit 204A (204B) shown in FIG. 24 or 25.
For the sake of convenience, it is assumed that interference occurs in one direction from H waves to V waves. A coefficient representing the degree of interference is represented by .alpha., and the functions representing modulated waves are represented by V () and H ().
From the functions (25) and (30) in FIG. 27, it is understood that the following equations must be satisfied to perform inter-cross wave interference compensation: EQU H (f.sub.H1 +f.sub.H2 -f.sub.V3 -f.sub.V4)=H (f.sub.H1 +f.sub.H2 -f.sub.H3 -f.sub.H4)
i.e., EQU f.sub.V3 +f.sub.V4 =f.sub.H3 +f.sub.H4 ( 1)
Since the carrier waves are synchronized between the transmitting side 100 and the receiving side 200, the following equations are given: EQU f.sub.V1 +f.sub.V2 -f.sub.V3 -f.sub.V4 =0 (2) EQU f.sub.H1 +f.sub.H2 -f.sub.H3 -f.sub.H4 =0 (3)
From equation (2), the following equation can be obtained. EQU f.sub.V1 +f.sub.V2 =f.sub.V3 +f.sub.V4 ( 4)
From equation (3), the following equation can be obtained. EQU f.sub.H1 +f.sub.H2 =f.sub.H3 +f.sub.H4 ( 5)
Substitution of equations (4) and (5) into equation (1) gives the following equation. EQU f.sub.V1 +f.sub.V2 =f.sub.H1 +f.sub.H2 ( 6)
Accordingly, the above-described synchronization of carrier waves can be performed by adjusting the local oscillators 105A, 105B, 106A and 106B in the transmitting side 100.
However, the synchronization of carrier waves by local oscillators 105A, 105B, 106A and 106B considerably increases costs. In addition, this is unfavorable in terms of maintenance such as fine adjustment performed when synchronization is lost.